Inter-cell interference is one of the main factors that limit spectrum efficiency of wireless communication systems, in particular for users located at the cell edge of a cellular system.
In second generation systems, like GSM (Global System for Mobile Communications), inter-cell interference is kept under control by limiting the usage of the frequency resources in the spatial domain by means of a reuse factor larger than one.
In third generation systems, like UMTS (Universal Mobile Communications System), where the frequency reuse factor is unitary, inter-cell interference for cell edge users is mitigated by means of the soft/softer handover technique through connection of a user equipment to a plurality of NodeBs.
In order to solve the problem of the inter-cell interference in Fourth Generation (4G) systems, like LTE-A, it has been recently introduced the concept of Coordinated Multi-Point Transmission and/or reception, also known with the acronym CoMP. The basic idea is to have some form of coordination between the different radio base stations (called eNodeB in LTE-A) in order to limit the inter-cell interference or to turn the interfering signals into useful ones. In the first case data are transmitted only from one transmission point (i.e. the serving cell) and the other cells adapt their transmission in order to reduce the interference over specific time/frequency transmission resources. These schemes are known in the 3GPP terminology as Coordinated Scheduling/Coordinated Beamforming (CS/CB). In the second case data are transmitted to the user from multiple cells so that the interference is turned into a useful signal. This second class of CoMP schemes are denoted as Joint Processing (JP) according to the 3GPP terminology.
Coordination among cells can occur at different levels of the protocol stack: like for example at the physical layer level (L1 coordination), at scheduling level (L2 coordination) or at Radio Resource Management level (L3 coordination). In any case the objective of CoMP is to proactively reduce, or ideally null, the inter-cell interference and thus to provide a uniform QoS (e.g. throughput) for all the served users irrespective of their position within the cell. WO2010/116340 discloses a system and method for reducing inter-cell interference within a CoMP cell (term used to identify a set of eNodeBs serving different cells), wherein the CoMP cell controller gathers scheduling information on adjacent CoMP cells in order to schedule data transmission to user equipments (UEs) within the controlled CoMP cell.
Coordination between cells has clearly a cost represented by the overhead deriving from the exchange of information (data, control and measurements) over the radio interface and over the backhauling links between the cooperating cells.
Patent application WO 2010/036158 provides a user grouping method to tradeoff the CoMP downlink capacity and the required uplink feedback. A channel element, such as the large scale fading, is measured for each distributed transmitter and user equipment (UE) pair based on reference signals, and the network receives this information as feedback in the uplink. The CoMP network determines a maximum cross interference level αreq that is affordable, based on the available uplink capacity for feedback. If this maximum cross interference level αreq is exceeded, geographically separate UEs are divided into as few groups as possible, with UEs within each group separated, and the groups are allocated different time/frequency resource blocks. The grouping is done with the constraint that the cross interference does not exceed αreq. This solution does not reduce too much the network overhead due to CoMP, since it requires the network to evaluate if the maximum interference level is exceeded, which means that the network shall receive a huge amount of data (reducing uplink capacity) and shall implement further calculation to implement user grouping.
Moreover, CoMP techniques requires the UE to search and identify cells that are suitable to implement coordinated transmission and reception. This has the drawback of increasing UE complexity (as more hardware for processing and storing is needed) and of stressing battery consumption (as several measurements are required). In order to solve this problem WO2010/039066 discloses a CoMP system wherein a UE is served by several subcells belonging to one serving CoMP cell and wherein common channel signals are identically transmitted by all subcells in a CoMP cell. In this system UE executes cell searching only within target CoMP cells by comparing a CoMP cell quality performance measure with a threshold value that can be provided by the network.
This solution reduces the UE complexity, not the network one, which still suffers of the network overhead provided by coordination of cells/subcells in CoMP systems.